When the electrical impedance of a human thorax is measured a variation synchronous with the heart activity can be obtained. The time derivative of this variation is called the Impedance CardioGram (ICG). Numerous studies during the last decades have demonstrated that the origin of this ICG-signal is too complicated to make an interpretation of the
Up to date little is known about the behaviour of ISTI in various physiological and clinical circumstances. There is a need for observational research to reveal the behaviour of the ISTI under these circumstances. This behaviour will also indicate the value of ISTI as an independent addition to cardiovascular reflex tests designed to diagnose autonomic nervous dysfunction [7]. The aim of the present study was to investigate the respiratory variation in ISTI (and RR-interval) at rest and during stimulated deep breathing in a group of patients suffering from Parkinson’s disease (PD) and to compare these variations with a group of healthy control subjects.
Figure 2 shows an example of a simultaneous recording of an ICG and an ECG signal in the same subject. The ICG-signal is the first time-derivative of the impedance variations,
Recordings were made using an impedance cardiograph custom designed and built at the Department of Physics and Medical Technology, VU University Medical Center, Amsterdam, the Netherlands. The signals were AD-converted using an ADInstruments PowerLab 8SP and processed on a personal computer using ADInstruments Chart (version 5.1.1) for Windows.
This calculation resulted in the outcome measures RR-VAR and ISTI-VAR in the two groups of subjects, during normal breathing at rest and during stimulated breathing. Significance of differences within the groups was tested using the paired Wilcoxon signed rank test and between the two groups using the unpaired Mann-Whitney test. A level of p < 0.05 was considered to be significant. The study was approved by the local medical ethics committee and all subjects gave their written informed consent.
No differences were found both in RR-interval between the two groups, or within the groups between the two conditions: normal breathing at rest and deep, stimulated breathing at 0.1 Hz. The results are listed in the table 1.
RR-interval ± S.D. [ms] in the group of patients with Parkinson’s disease and in the group of control subjects while breathing at rest and during stimulated, deep breathing at 0.1 Hz.
subjects | RR [ms] at rest | RR [ms] stimulated |
---|---|---|
controls (24) | 990 ± 180 | 990 ± 170 |
patients (19) | 940 ± 170 | 940 ± 170 |
No differences were found also in ISTI-interval between the two groups, or within the groups between the two conditions. The results are listed in the table 2.
ISTI-interval ± S.D. [ms] in the group of patients with Parkinson’s disease and in the group of control subjects while breathing at rest and during stimulated, deep breathing at 0.1 Hz.
subjects | ISTI [ms] at rest | ISTI [ms] stimulated |
---|---|---|
controls (24) | 145 ± 14 | 147 ± 10 |
patients (18) | 143 ± 17 | 147 ± 19 |
The variability, however, of the RR-interval, as indicated by RR-VAR, was higher during deep metronomic breathing than in the resting condition for both the control group (p < 0.01) and the patient group (p < 0.025). Although the RR-VAR increased during deep breathing in both groups, the RR-VAR was significantly higher in the control group than in the PD group (p < 0.025). There was no significant difference in RR-VAR between both groups breathing at rest (p > 0.05). The results are listed in table 3.
Variability in RR-interval (RR-VAR) ± S.D. [%] in the group of patients with Parkinson’s disease and in the group of control subjects while breathing at rest and during stimulated, deep breathing at 0.1 Hz.
subjects | RR-VAR [%] at rest | RR-VAR [%] stimulated |
---|---|---|
controls (24) | 19 ± 10 | 31 ± 12 |
patients (19) | 17 ± 7 | 23 ± 10 |
RR-VAR in both groups and in both conditions are presented graphically in figure 3.
The variability in ISTI at rest, as indicated by ISTI-VAR, was higher in patients than in control subjects (p < 0,05). The variability increased significantly during deep breathing in the control group (p < 0.01) but did not change significantly in the patients group. The results are listed in table 4.
Variability in ISTI (ISTI-VAR) ± S.D. [%] in the group of patients with Parkinson’s disease and in the group of control subjects while breathing at rest and during stimulated, deep breathing at 0.1 Hz.
subjects | ISTI-VAR [%] at rest | ISTI-VAR [%] stimulated |
---|---|---|
controls (24) | 23 ± 15 | 34 ± 17 |
patients (18) | 44 ± 40 | 38 ± 20 |
ISTI-VAR in both groups and in both conditions are presented graphically in figure 4.
The observations in patients with PD demonstrate that measurement of ISTI can be useful as an addition to cardiovascular reflex testing in diagnosing autonomic dysfunction. No significant differences were found in RR and ISTI between the PD patient group and the control group at rest or during stimulated, deep breathing. The
ISTI variability was significantly higher in PD patients than in controls during normal breathing at rest. It increased during deep metronomic breathing in the control group, but not in the patient group as it was already high during normal breathing. ISTI is directly related to the pre-ejection period (PEP) [5], which is an index of contractility and only influenced by sympathetic but not by parasympathetic activity in humans [12, 13, 14]. Apparently the data show that the variability in sympathetic activity at rest was higher in patients than in controls. A possible explanation is that, in PD patients, the sympathetic system has to compensate for the decreased response of the parasympathetic system to level blood pressure variations caused by breathing. During stimulated deep breathing, the ISTI variability increased significantly in controls but not in PD patients. This result can be explained by assuming that the high ISTI variability in PD patients was already at a maximal level at rest. This limited compensatory mechanism of the sympathetic system also explains the reduced RR variability in PD patients during deep breathing. This mechanism appears to be at a maximal level in normal breathing and fails in compensating during deeper breathing. As a consequence, the RR variability is limited in PD patients in comparison to controls.
It is not likely that these observations simply resulted from a normal difference in the mechanics of the ventilation between the two groups [15, 16]. If the lower RR variability in PD patients during stimulated breathing were the result of a poor breathing performance, the ISTI variability would have been lower as well. If the higher ISTI variability in PD patients in rest were the consequence of deeper breathing in rest, the RR variability in rest would have been higher as well. Neither was observed.
From these results it is concluded that ISTI is a practical, additional and independent parameter that can be used to assist other tests in evaluating autonomic control of the heart in Parkinson’s disease. The necessary technical equipment is relatively inexpensive and simple to operate. The measurement of ISTI is non-invasive and can be used for monitoring purposes. The measurement, however, is not limited to a clinical environment or to clinical applications.